The invention concerns auto injectors having separable front and rear housings where the front housing is arranged to receive and hold a container subassembly, such as a syringe, and the rear housing comprises a power pack unit arranged to act on a plunger in the container subassembly such that a medicament can be expelled. WO-2010/076569 discloses an injector in which a delivery member cover can be locked against retraction such that the delivery member is not exposed. This provides for a certain degree of user friendliness and safety. However, the arrangement of that prior art has a drawback in that the power pack unit may unintentionally be released. If a user, for some reason, pushes the firing button, the plunger rod will be released and fly off due to the force of the main drive spring, which may be rather strong. Since in many cases, the front part and the rear part are provided by one supplier and the container part is provided by another supplier and final assembly of these parts is done by a third part prior to reaching an end consumer, it is of utmost importance that the different parts are delivered in a “ready-to-assemble” condition. The power pack unit might however be unintentionally released during handling or transportation, thereby releasing a plunger rod. This is first of all dangerous since the forces involved are considerable; secondly it requires quite an amount of extra work to re-assemble the power pack unit.